Alpha Centauri B
Star
Alpha Centauri B (Toliman) — an orange K1V dwarf, slightly smaller and cooler than the Sun, locked in a 79.8-year orbit with Alpha Centauri A. Together the pair forms the heart of the closest star system to us.
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Key facts
Mutual orbit
79.8 Earth years
29,133 Earth days
Mass
1.81 × 10³⁰ kg
0.909 M☉
Radius
597,676 km
86% of the Sun's radius
Surface temperature
5,207 K (4,934 °C)
Spectral type
K1V (orange dwarf — smaller and cooler than the Sun)
Science
Physical
- Mass
- 1.81 × 10³⁰ kg · 0.909 M☉
- Radius
- 597,676 km · 86% of the Sun's radius
- Density
- 2,022 kg/m³ · 2× water
- Computed from mass and radius
- Spectral type
- K1V (orange dwarf — smaller and cooler than the Sun)
Motion through space
- Note
- The motion of the pair's common centre of mass, computed as the mass-weighted mean of its stars' catalogued motions.
- Proper motion
- 3.7 arcseconds per year · its drift across our sky
- SIMBAD — 2007A&A...474..653V
- Radial velocity
- -18.6 km/s · approaching the Sun
- SIMBAD — 2007A&A...474..653V
- Speed across our line of sight
- 23.7 km/s
- Computed from proper motion and parallax
- Speed through space, relative to the Sun
- 30.1 km/s
- Computed from proper motion, parallax and radial velocity
Orbit (around the system barycenter)
- Semi-major axis
- 12.6 AU
- From the published visual-binary orbit (P = 79.76 yr, e = 0.519)
- Eccentricity
- 0.5195
- From the published visual-binary orbit (P = 79.76 yr, e = 0.519)
- Orbital period
- 79.76 years
- From the published visual-binary orbit (P = 79.76 yr, e = 0.519)
Rotation
- Equatorial period
- 38.7 days
- Note
- Rotation ≈ 39 days, inferred from activity monitoring
Composition
- By mass
- Spectroscopically similar to A: hydrogen/helium, metal-rich relative to the Sun. More magnetically active, with a stellar activity cycle of roughly 8 years
Temperature
- Surface
- 5,207 K (4,934 °C)
Radiation & temperature
- Luminosity
- 1.91 × 10²⁶ W · 0.498 L☉
- Surface temperature
- 5,207 K (4,934 °C)
- Peak wavelength
- 557 nm · green-yellow visible light
- Computed (Wien's law) from the surface temperature
Discovery & history
- Method
- Telescope
- Name
- Toliman — the IAU proper name (approved 2018), from an Arabic name for the constellation's stars sometimes rendered 'the ostriches'.
Resolved as a separate star from Alpha Centauri A by Jean Richaud in December 1689 — observing from Puducherry, India, he found the 'single' bright star of Centaurus to be a close pair. The pair's 80-year waltz has since been tracked through more than four complete orbits.
Images
Images from open archives · credit shown per image